1. Field of the Invention
The present invention relates to zoom lenses for use in single-lens reflex cameras or the like and, more particularly, to zoom lenses having a high zoom ratio with inclusion of a focal length range from the wide-angle region and up to a middle-telephoto region. Still more particularly, the invention relates to such zoom lenses which are equipped with a mechanism responsive to accidental vibrations of the camera for compensating the shake of an image to be photographed, that is, which have the image stabilizing function.
2. Description of Related Art
Up to now, a great number of standard zoom lenses whose range covers both sides of the focal length equal to the diagonal length of the image frame have been proposed. In view of the zoom configuration alone, there are a wide variety of types beginning with the 2-unit one and extending to the 3-, 4- and 5-unit ones. Of these, although the 2- or 3-unit type is suitable to reduction of the size of the lens system to a compact form, a high range is difficult to realize. For the 4-unit type zoom lens, to increase the zoom ratio in such a manner as to keep a high optical performance, there are problems in that the total zooming movement of each lens unit has to be much longer and that each lens unit has to be made up from a somewhat greater number of lens members. So, in order to realize a zoom lens which has a range of covering the wide-angle region up to a middle-telephoto region, is constructed in a compact form, while still permitting a good optical performance to be achieved, the 5-unit type has been proposed. Such an increase of the number of movable lens units makes their air separations in appropriate ways to facilitate correction of all aberrations as produced during zooming. In the present state of art of zoom lenses, therefore, a much desired increase of the zoom ratio becomes possible to realize.
Conventional examples which employ the 5-unit type in constructing a zoom lens and succeed in giving it a high zoom ratio particularly in respect of the range of focal lengths of from the wide-angle region to a middle-telephoto region are disclosed in, for example, Japanese Laid-Open Patent Applications Nos. Sho 63-189819 and Sho 60-39613.
In the Japanese Laid-Open Patent Application No. Sho 63-189819 taken as a first conventional example, the zoom lens comprises, in order from an object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power and a fifth lens unit having a negative refractive power, totalling five lens units, with the result mainly of a zoom ratio of a little less than 4.
In the Japanese Laid-Open Patent Application No. Sho 60-39613 taken as a second conventional example, the zoom lens comprises, in order from an object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a negative refractive power and a fifth lens unit having a positive refractive power, totalling five lens units, with the result mainly of a zoom ratio of a little less than 5.
Meanwhile, as the camera vibrates accidentally, the zoom lens forms a shaking image. To take photographs of good quality, therefore, means must be provided for compensating the vibrations. In particular, a lens unit constituting a part of the optical system is moved in the nearly perpendicular directions to the optical axis, as disclosed in, for example, Japanese Laid-Open Patent Applications Nos. Hei 2-35406 and Hei 8-136862.
The Japanese Laid-Open Patent Application No. Hei 2-35406 as a third conventional example discloses a preferred embodiment suited for application mainly to leaf-shutter-type cameras. The disclosed zoom lens is of the 3-unit type, comprising, in order from an object side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a negative refractive power. One of these lens units is made to move in the nearly perpendicular directions to the optical axis, thus compensating the shake of an image to be photographed.
The Japanese Laid-Open Patent Application No. Hei 8-136862 as a fourth conventional example discloses an embodiment suitable for application mainly to the standard zoom lenses for the single-lens reflex camera. The zoom lens is of the 4-unit type, comprising, in order from an object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power. The second lens unit is made to move in the nearly perpendicular directions to the optical axis, thus compensating the shake of a image to be photographed.
The aforesaid first conventional example has realized a compact high-range zoom lens with the zoom ratio being about 4. However, there is a difficult problem of sufficiently shortening the focal length for the wide-angle end. In that case, as the zoom ratio increases to 4 or higher, the difficulty increases rapidly, producing another problem.
The second conventional example has realized a zoom lens which has as high a range as about 5. However, there is a very difficult problem of simultaneously fulfilling the requirement of keeping the compact form and of maintaining a good stability of optical performance at a high level throughout the entire zooming range. For the lens units, too, there is a problem in that the residual aberrations tend to increase. Therefore, as the lens units move, the sharp image plane of the entire lens system is caused to shift and the range of variation of aberrations increases greatly. To remedy these defects, on the other hand, the zoom lens cannot be manufactured economically.
Meanwhile, even in the above-described conventional examples of providing the zoom lens with the prior known means for stabilizing the image against vibrations, there are the following problems.
The third conventional example is advantageous for incorporating the image stabilizing mechanism in the mechanical mounting for that zoom lens which is adapted to be used mainly in the leaf-shutter-type camera. When the zoom lens configuration disclosed here is applied to the interchangeable lenses for the single-lens reflex camera, on the other hand, a problem arises in that the back focal distance is not long enough to create a space that allows the quick-return mirror to operate.
The fourth conventional example is adapted for application mainly to the standard zoom lenses for the single-lens reflex camera. Although the aim of the fourth conventional example is to incorporate the image stabilizing mechanism to such lenses, the use of the 4-unit type leads to a difficult problem of realizing the increase of the range, as described above. Based on the features of the image stabilizing zoom lens, a new optical system may be made up in combination with the second conventional example. Although such a measure seems to be one solution of the problems, a new problem arises in that the image stabilizing mechanism becomes difficult to coexist with the focusing mechanism. In more detail, suppose that the second conventional example has realized that the negative second lens unit moves in the directions nearly perpendicular to the optical axis to compensate the vibrations, then the adequate amplitude of vibrations can be compensated by a relatively small amount of that lens unit as is understandable from the description of the fourth conventional example. As far as the introduction of the means for compensating the vibrations is concerned, therefore, it becomes possible to form the mounting mechanism to a relatively small size.
However, in the zoom lens described in the second conventional example, if the focusing provision is made at the negative second lens unit, as disclosed in, for example, Japanese Laid-Open Patent Application No. Hei 5-119260, the minimum object distance can be shortened enough. As is also known, the variation of all aberrations can be corrected well. Another focusing method is known too, which axially moves both of the positive first and negative second lens units. In any case, for the zoom lens like that of the second conventional example, it is desirable to choose at least the second lens unit for the focusing purpose.
Therefore, when the vibration compensating mechanism of the fourth conventional example is applied to the type of zoom lens of the second conventional example, the negative second lens unit not only is associated with the mechanism for compensating the vibration but also has to be equipped with a focus adjusting mechanism. This leads to an obstacle in realizing an improved compact form of the mechanical mounting for the optical system. Also, in the zoom lens of the type of the second conventional example, the negative second lens unit tends to have a relatively large diameter. In this point, too, even any combinations of these conventional examples have hardly been able to realize a zoom lens having an improved compact form with a high zoom ratio and equipped with the mechanism for compensating the vibrations.